In pitch controlled wind turbines, the pitch of the wind turbine blades is adjusted in response to the wind conditions, and in order to obtain a desired energy production. As the wind turbine blades rotate during operation, the loads on each wind turbine blade changes, e.g. due to wind shear, tower passage, turbulence, etc. This may cause asymmetric load distribution among the wind turbine blades. This is undesirable, since it leads to high loads on the rotor, the drive train, etc. In order to avoid this, an individual pitch control strategy is sometimes used. According to an individual pitch control strategy, the pitch angles of the individual blades are adjusted in order to take conditions at the position of each blade into account.
When a halt command or shutdown command is generated for a pitch controlled wind turbine, e.g. due to an emergency situation, the wind turbine blades are normally moved to a feathered position, i.e. their pitch angles are changed until the wind turbine blades are in a position where they no longer catch the wind. It is often desirable to move the wind turbine blades as quickly as possible to the feathered position.
However, in the case of individually pitched wind turbines, the pitch angles of the wind turbine blades are not identical when the halt command or shutdown command is received. On the contrary, the pitch angle of each wind turbine blade has been adjusted in such a manner that it takes the conditions prevailing at the exact position of the blade in question into account. If all the wind turbine blades are simply moved as quickly as possible towards the feathered position when the halt command or shutdown command is received, the mutual differences in the pitch angles will remain as the wind turbine blades continue rotation in the rotor plane, while moving towards the feathered position. Thereby the wind turbine blades are moved away from the positions which dictated the adjustments in the pitch angles, but the adjustments are not altered. This may lead to asymmetric loads on the wind turbine blades which are even worse than would be the case if the pitch angles of the wind turbine blades were simply identical. It is therefore desirable to provide a control strategy during shutdown of an individually pitched wind turbine, which reduces the asymmetric load on the wind turbine blades.
EP 2 290 232 discloses a wind turbine pitch angle control device configured to carry out individual pitch control. When a wind turbine halt command is input, the pitch angles of the wind turbine blades are matched, and then the pitch angles are moved to a feathered position. When the wind turbine halt command is input, a representative wind turbine blade may be identified on the basis of the pitch angle from among the plurality of wind turbine blades. The pitch angle of the representative wind turbine blade and the pitch angles of the other wind turbine blades are then matched, and then the pitch angles of the wind turbine blades are moved to the feathered position. The representative wind turbine blade may be the wind turbine blade whose pitch angle is closest to the feathered position.
The control device of EP 2 290 232 reduces the asymmetric loads on the wind turbine blades during shutdown, because the pitch angles are aligned before the wind turbine blades are moved to the feathered position. However, the shutdown process is delayed because the aligning must take place before the movement towards the feathered position is commenced. Furthermore, the same shutdown strategy is used each time a halt command is input, even though this shutdown strategy may not be appropriate in all situations.